Polyesters such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) are known to possess good chemical stability, physical and mechanical strength, durability, heat resistance, chemical resistance, weather resistance and electrical insulation property; and, therefore, have been widely used in manufacturing various articles including medical devices, capacitors, packaging and labelling materials, photographic film and magnetic recording media.
Polyester films having embossed surface are often prepared so as to improve surface properties of the film. The embossing can be typically formed by incorporating inert particles such as particles of calcium carbonate, silicon oxide, aluminum oxide and kaolin into the raw materials during the production of the film, or by forming inorganic particles in situ.
Japanese Patent Laid-open Publication No. 214734/1990 discloses the use of light calcium carbonate particles and .alpha.-, .gamma.- or .delta.-alumina particles to enhance the scratch resistance of a polyester film. Further, Japanese Patent Laid-open Publication No. 151231/1992 describes the use of a mixture of light calcium carbonate particles and a colloidal silica to impart scratch and abrasion resistance to a polyester film.
However, the above methods have various problems. For example, light calcium carbonate particles tend to wear out easily due to their low hardness, and when a film containing them is used as the base film of a video tape, it is easily scratched, e.g., by a guide roll during a magnetic layer coating process thereof and by a calendering roll during a calendering process. The scratching of the base film may cause the drop-out phenomena of the magnetic tape.
Moreover, calcium carbonate particles have a low affinity to a polyester resin, which may lead to the formation of voids during a draw process of the film and lowering of the dielectric strength of the film. Such voids may cause the film surface to abrade and the calcium carbonate particles to fall off, and accordingly, such polyester film is easily damaged when used as a magnetic tape. Also, a polyester film having a low dielectric strength is not suitable in preparing an electric product, e.g., an capacitor.
Further, the use of alumina particles brings out the problems that a stable slurry containing a high alumina content is difficult to prepare due to the thickening effect of the alumina particles. Silica particles, on the other hand, are used to improve the clarity of a polyester film, but they have a low affinity to the polyester resin and induce the formation of voids during a draw process as well as the formation of rough protrusions on the film during a winding process.